From a a tech standpoint, Roblox definitely has a dramatic lead over vanilla Minecraft's tools for creating arbitrary added content.

That said, if you factor in the tools the Minecraft community have built (namely the Force modloader, and the Bukkit multiplayer plugin system), it becomes a lot harder to say. On one hand, the barrier to entry in Roblox is practically non-existent (I remember screwing around with lua scripts while I was still in elementary school), while on the other hand I've seen far more technically impressive content come out of the Minecraft modding world.